<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      border: solid;
    }

    .container {
      width: 800px;
      height: 800px;
      display: flex;
      flex-flow: column;
    }

    .container .left {
      flex: 0 0 200px;
    }

    .container .center {
      flex: auto;
    }

    .container .right {
      flex: 0 0 200px;
    }
  </style>
</head>

<body>
  <div class="container">
    <div class="left">左</div>
    <div class="center">中</div>
    <div class="right">右</div>
  </div>
</body>

</html>